1. Field of the Invention
The present invention relates to a logarithmic amplifier and, more particularly, to a logarithmic amplifier designed to make it faster.
2. Description of the Prior Art
Current signals having a very wide range, such as output signals from a photodetector which reads, for example, a high gradation image, are often subjected to logarithmic amplification in order to make the current signals easy to handle, by compressing the range. As disclosed in Unexamined Japanese Patent Publn. No. Hei-2(1990)-277181, one of known logarithmic amplifiers for carrying out logarithmic amplification has an operational amplifier, an element for logarithmic conversion purposes connected to a feedback circuit of the operational amplifier, an oscillation prevention circuit having a capacitor connected in parallel with the logarithmic conversion element, and a control circuit for controlling the oscillation prevention circuit in such a way that the amount of feedback by way of the oscillation prevention circuit is reduced as an input current to the operational amplifier becomes smaller.
The logarithmic amplifier of this type is designed so as to achieve stable operation and high-speed response by increasing the amount of feedback of high frequency components when the operation of the amplifier is prone to become unstable when receiving a large input current. Contrary to this, if the logarithmic amplifier has no problem of operation stability but a problem of response characteristics when receiving a small input current, the amount of feedback of the high frequency components is suppressed. As a result of this, the stable operation and improved high-speed response characteristics of the circuit can be obtained at the same time.
To increase the high-speed response of such a conventional logarithmic amplifier to a much greater extent, it is necessary to construct an operational amplifier itself of an operational amplifier having superior high-speed characteristics. As an operational amplifier having superior high-speed characteristics, a bipolar input type amplifier is known. The conventional logarithmic amplifier has a circuit configuration for reducing the amount of feedback by way of the oscillation prevention circuit as an input current becomes smaller. An input bias current causes an error in the conventional logarithmic amplifier having the previously mentioned configuration. In practice, there is no alternative but to use an FET input type operational amplifier having a small input bias current in view of accuracy, and it has been impossible to use a bipolar input type operational amplifier. For this reason, it has been difficult for the conventional logarithmic amplifier to obtain sufficient high-speed response characteristics.